Blocks are spontaneously destroyed. For example, look at what happened to this blacksmith. The same bug also destroyed some fences deep underground.

This makes Minecraft pretty much unplayable, because I never know when the glitch might destroy part of a fence, letting all my cows out, or letting creepers into what should be a secure area. Maybe it will let zombies into my villager commune. Maybe it will destroy part of a bridge in the Nether and I'll fall into the lava. Maybe it will delete a chest with 27 shulker boxes in it.

It's a new world generated in 1.13, and the seed is 17451728208755585